| 1. |
- Sannel, A. Britta K., 1968-
(författare)
-
Temporal and spatial dynamics in subarctic peat plateaus and thermokarst lakes
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Permafrost peatlands are widespread at high northern latitudes and are important soil organic carbon reservoirs. A future warming in these areas, as suggested by global climate models, can cause thawing and increased ground subsidence (thermokarst), resulting in changes in surface hydrology and ecosystem functioning. The aim of this thesis is to increase our knowledge of temporal and spatial dynamics in subarctic peat plateaus with interspersed thermokarst lakes in order to better understand how these ecosystems respond to climate change. Detailed plant macrofossil and carbon/nitrogen ratio analyses of two peat plateaus located in the continuous and northern discontinuous permafrost zones in west-central Canada show that permafrost conditions have been stable since permafrost developed around 5600–4500 cal yr BP. Peat plateaus act as carbon sinks over time. The lack of wet phases since the plateaus formed, despite several local fires, suggests that this type of peatlands have been negligible as methane sources throughout most of their history, representing a negative net radiative forcing on climate. Thermokarst lakes are common features in peat plateaus across the northern permafrost region. A time-series analysis of aerial photographs and high resolution satellite images in three peat plateau/thermokarst lake complexes along a climatic and permafrost gradient shows that where the mean annual air temperature (MAAT) is below -5ºC and ground temperatures are -2ºC or colder, only minor changes in thermokarst lake extent have occurred from the mid 1970s until the mid 2000s. During the same time interval extensive lake drainage and new lake formation has taken place where the MAAT is ca -3ºC and the ground temperature is close to 0ºC. In a future progressively warmer and wetter climate, permafrost degradation can cause significant impacts on landscape pattern and greenhouse gas exchange also in the vast peat plateaus presently experiencing stable permafrost conditions.
|
|
| 2. |
- Hamm, Alexandra, 1993-
(författare)
-
Permafrost Groundwater Dynamics : Modeling of vertical and lateral flows in the active layer across multiple scales
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hydrological processes in the Arctic are profoundly influenced by the presence of permanently frozen ground, known as permafrost. Conversely, permafrost is greatly affected by hydrological changes resulting from climate change. Understanding and accurately representing the processes causing permafrost thaw is essential for evaluating the consequences of climate change on permafrost landscapes. In this thesis, I explore how water movements in permafrost landscapes affect the thermal state of the ground and the potential of groundwater flow to transport both heat and solutes. As groundwater is inherently difficult to observe in field experiments, the main method in this thesis is simulating permafrost dynamics with a state-of-the-art physics-based numerical model. Modeling allows investigating these dynamics in both space and time. Results show that an increase in summer rainfall and the associated vertical movement of water in the soil causes opposing effects in the ground temperature response. While enhanced summer rainfall likely leads to a warming in continental permafrost landscapes, in maritime landscapes it may cause a cooling of the ground. This is governed by the effects of rainfall on the hydrothermal properties of the soil and how efficiently it conducts and stores energy.Lateral water movement was found to substantially affect soil moisture distribution along a hillslope underlain by continuous permafrost. Soil moisture is important in the context of the hydrothermal properties within a hillslope but also for the capability of the ground to transport solutes. High soil moisture leads to higher soil hydraulic conductivity and therefore affects how fast solutes such as dissolved organic carbon can be transported with the groundwater. Depending on the vertical location of solutes within the soil, this determines the travel time of solutes in the groundwater towards surface water recipients. Additionally, depending on the rate at which air temperatures will increase in the future, permafrost carbon may experience different modes of lateral transport and residence times in the soil. This thesis highlights the complex interplay between permafrost and hydrology and why it is important to study them as a coupled system in order to fully understand the impacts of climate change on the fate of permafrost.
|
|
| 3. |
- Hugelius, Gustaf, 1980-
(författare)
-
Quantity and quality of soil organic matter in permafrost terrain
- 2011
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High latitude terrestrial ecosystems are considered key components in the global carbon (C) cycle and hold large reservoirs of soil organic carbon (SOC). Much of this is stored as soil organic matter (SOM) in permafrost soils and peat deposits and is vulnerable to remobilization under future global warming. While the large size and potential vulnerability of arctic SOM reservoirs is recognized, detailed knowledge on its landscape partitioning and quality is poor. This thesis describes total storage, landscape partitioning and lability of SOM stored in permafrost areas of Canada and Russia. Detailed studies of SOC partitioning highlight the importance of especially permafrost peatlands, but also of O-horizons in moist tundra soils and cryoturbated soil horizons. A general characterization of SOM in an area of discontinuous permafrost shows that >70% of the SOC in the landscape is stored in SOM with a low degree of decomposition. Projections of permafrost thaw predict that the amount of SOC stored in the active layer of permafrost soils in this area could double by the end of this century. A lateral expansion of current thermokarst lakes by 30 m would expose comparable amounts of SOC to degradation. The results from this thesis have demonstrated the value of high-resolution studies of SOC storage. It is found that peat plateaus, common in the sporadic and discontinuous permafrost zones, store large quantities of labile SOM and may be highly susceptible to permafrost degradation, especially thermokarst, under future climate warming. Large quantities of labile SOM is also stored in cryoturbated soil horizons which may be affected by active layer warming and deepening. The current upscaling methodology is statistically evaluated and recommendations are given for the design of future studies. To accurately predict responses of periglacial C pools to a warming climate detailed studies of SOC storage and partitioning in different periglacial landscapes are needed.
|
|
| 4. |
- Kaislahti Tillman, Päivi, 1958-
(författare)
-
Holocene climate and environmental change in high latitudes as recorded by stable isotopes in peat deposits
- 2012
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, stable carbon and oxygen isotopes in α-cellulose isolated from Sphagnum fuscum moss remains were used as climate proxies. The main focus was to implement the methods in records from high latitude peatlands in the Northern Hemisphere (west-central Canada and north-eastern European Russia), reconstruct palaeoclimate of the studied regions during the Holocene, and evaluate the compatibility of results with other proxy records, especially tree-ring isotope time-series. The variation of stable carbon and oxygen isotope ratios (δ13C, δ18O) in different moss plant components was investigated and differences between untreated plants and α-cellulose extracts were evaluated. The impact of peat decay on the stable isotope proxies was studied by colorimetric and chemical (C/N) methods. Temperature reconstructions were developed based on the statistically significant relationship between δ13C and modern summer temperature records. Wet/dry periods were derived from a combination of δ18O records, macrofossil analysis, and a peat humification record in west-central Canada. A tentative reconstruction of snow depth in north-eastern European Russian tundra and northern taiga was based on δ18O records. The most promising result of the thesis is that stable carbon isotope variability in α-cellulose isolated from Sphagnum fuscum stems can be used to reconstruct and quantify palaeotemperatures several millennia back in time and to reveal both long-term and rapid climate shifts from peat archives.
|
|
| 5. |
- Lindgren, Amelie, 1987-
(författare)
-
Northern Permafrost Region Soil Carbon Dynamics since the Last Glacial Maximum : a terrestrial component in the glacial to interglacial carbon cycle
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- At the Last Glacial Maximum (LGM), after ~100,000 years of relatively cold temperatures and progressively lower atmospheric carbon dioxide (CO2) concentrations, CO2 levels reached ~180 ppm, which is less than half of what we see today in a much warmer world (~400 ppm). Although much of this increase since the LGM is due to human-induced emissions, about 100 ppm of this increase can be attributed to natural variations seen over glacial to interglacial cycles. The sources for this natural CO2 rise remain unclear despite considerable efforts to constrain its origin. This thesis attempts to describe and quantify the role of soil carbon in this context, with emphasis on the permafrost hypothesis, which states that a shift from glacial to interglacial conditions released permafrost soil carbon to the atmosphere during the deglaciation. We present empirical estimates of the change in the Northern permafrost area between the LGM and present, and the associated soil carbon stock changes. We also partition these soil carbon stock changes at millennial intervals to capture not only the size but the timing of change. We find that the soil carbon stocks north of the Tropics decreased after the LGM to reach a minimum around 10,000 years ago, after which stocks increased to more than compensate for past losses. This may present part of a solution to untangle the marine and atmospheric 13C record, where the marine records suggest that the terrestrial carbon stock has grown since the LGM, while the atmospheric record also indicates terrestrial losses. To estimate the mineral soil carbon stocks, we have relied on vegetation reconstructions. Some of these reconstructions were created with a novel data-driven machine learning approach. This method may facilitate robust vegetation reconstruction when evidence of past conditions is readily available. Results in this thesis highlight the importance of permafrost, loess deposits and peatlands when considering the soil carbon cycle over long time scales.
|
|
| 6. |
- Palmtag, Juri, 1980-
(författare)
-
Landscape partitioning and burial processes of soil organic carbon in contrasting areas of continuous permafrost
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Recent studies have shown that permafrost soils in the northern circumpolar region store almost twice as much carbon as the atmosphere. Since soil organic carbon (SOC) pools have large regional and landscape-level variability, detailed SOC inventories from across the northern permafrost region are needed to assess potential remobilization of SOC with permafrost degradation and to quantify the permafrost carbon-climate feedback on global warming.This thesis provides high-resolution data on SOC storage in five study areas located in undersampled regions of the continuous permafrost zone (Zackenberg in NE Greenland; Shalaurovo and Cherskiy in NE Siberia; Ary-Mas and Logata in Taymyr Peninsula). The emphasis throughout the five different study areas is put on SOC partitioning within the landscape and soil horizon levels as well as on soil forming processes under periglacial conditions. Our results indicate large differences in mean SOC 0–100 cm storage among study areas, ranging from 4.8 to 30.0 kg C m-2, highlighting the need to consider numerous factors as topography, geomorphology, land cover, soil texture, soil moisture, etc. in the assessment of landscape-level and regional SOC stock estimates.In the high arctic mountainous area of Zackenberg, the mean SOC storage is low due to the high proportion of bare grounds. The geomorphology based upscaling resulted in a c. 40% lower estimate compared to a land cover based upscaling (4.8 vs 8.3 kg C m-2, respectively). A landform approach provides a better tool for identifying hotspots of SOC burial in the landscape, which in this area corresponds to alluvial fan deposits in the foothills of the mountains. SOC burial by cryoturbation was much more limited and largely restricted to soils in the lower central valley. In the lowland permafrost study areas of Russia the mean SOC 0–100 cm storage ranged from 14.8 to 30.0 kg C m-2. Cryoturbation is the main burial process of SOC, storing on average c. 30% of the total landscape SOC 0–100 cm in deeper C-enriched pockets in all study areas. In Taymyr Peninsula, the mean SOC storage between the Ary-Mas and Logata study areas differed by c. 40% (14.8 vs 20.8 kg C m-2, respectively). We ascribe this mainly to the finer soil texture in the latter study area. Grain size analyses show that cryoturbation is most prominent in silt loam soils with high coarse silt to very fine sand fractions. However, in profiles and samples not affected by C-enrichment, C concentrations and densities were higher in silt loam soils with higher clay to medium silt fractions.
|
|
| 7. |
- Ryner, Maria, 1967-
(författare)
-
Past environmental and climate changes in northern Tanzania : Vegetation and lake level variability in Empakaai Crater
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents palaeoenvironmental data from equatorial Africa covering two important time intervals; i) the warming period forming the Pleistocene/Holocene transition and ii) the last millennium. The Empakaai Crater, in northern Tanzania contains a lake from where sediment cores, spanning two time-slices 14.8-9.3 ka and 800-2000 AD, have been studied. Palaeoecological and palaeohydrological reconstruction is based on a multitude of proxies from the sediments, representing both catchment environment and the lakes aquatic ecosystem response. Between 14.8 and 10 ka the catchment vegetation and lake hydrology responded to both regional climate changes and local environment, but with different amplitude and frequency, reflecting temporal and spatial lags between the two systems. However, at c 10 ka both lake conditions and catchment vegetation showed drastic changes towards drier conditions. The record covering the last millennium reveals environmental changes related to climate and human activities. The catchment’s vegetation was affected by frequent fires, most probably human induced, while near shore vegetation responded to lake level fluctuation associated with rainfall variability. About 15 km from Empakaai Crater is an extensive abandoned irrigation system, the Engaruka complex, which was in active use between c 1400 AD and 1840 AD. By comparing a number of social and environmental factors potentially influencing the societal development at Engaruka it is shown that wet climate conditions have had positive effects on the societal development but also that dry climate conditions were not always disastrous to the society. The resemblance of the pollen taxa present is strong between the two time slices and pollen representing catchment conditions respond in similar manner in both records. The lake conditions are however very different between the two periods Thus the lake responds to both long and short term changes of variable amplitude, while the catchment vegetation seems to responds to high amplitude, low frequency changes.
|
|
| 8. |
- Siewert, Matthias Benjamin, 1985-
(författare)
-
High-resolution mapping and spatial variability of soil organic carbon storage in permafrost environments
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Large amounts of carbon are stored in soils of the northern circumpolar permafrost region. High-resolution mapping of this soil organic carbon (SOC) is important to better understand and predict local to global scale carbon dynamics. In this thesis, studies from five different areas across the permafrost region indicate a pattern of generally higher SOC storage in Arctic tundra soils compared to forested sub-Arctic or Boreal taiga soils. However, much of the SOC stored in the top meter of tundra soils is permanently frozen, while the annually thawing active layer is deeper in taiga soils and more SOC may be available for turnover to ecosystem processes. The results show that significantly more carbon is stored in soils compared to vegetation, even in fully forested taiga ecosystems. This indicates that over longer timescales, the SOC potentially released from thawing permafrost cannot be offset by a greening of the Arctic. For all study areas, the SOC distribution is strongly influenced by the geomorphology, i.e. periglacial landforms and processes, at different spatial scales. These span from the cryoturbation of soil horizons, to the formation of palsas, peat plateaus and different generations of ice-wedges, to thermokarst creating kilometer scale macro environments. In study areas that have not been affected by Pleistocene glaciation, SOC distribution is highly influenced by the occurrence of ice-rich and relief-forming Yedoma deposits. This thesis investigates the use of thematic maps from highly resolved satellite imagery (<6.5 m resolution). These maps reveal important information on the local distribution and variability of SOC, but their creation requires advanced classification methods including an object-based approach, modern classifiers and data-fusion. The results of statistical analyses show a clear link of land cover and geomorphology with SOC storage. Peat-formation and cryoturbation are identified as two major mechanisms to accumulate SOC. As an alternative to thematic maps, this thesis demonstrates the advantages of digital soil mapping of SOC in permafrost areas using machine-learning methods, such as support vector machines, artificial neural networks and random forests. Overall, high-resolution satellite imagery and robust spatial prediction methods allow detailed maps of SOC. This thesis significantly increases the amount of soil pedons available for the individual study areas. Yet, this information is still the limiting factor to better understand the SOC distribution in permafrost environments at local and circumpolar scale. Soil pedon information for SOC quantification should at least distinguish the surface organic layer, the mineral subsoil in the active layer compared to the permafrost and further into organic rich cryoturbated and buried soil horizons.
|
|
| 9. |
- Weiss, Niels, 1984-
(författare)
-
Permafrost carbon in a changing Arctic : On periglacial landscape dynamics, organic matter characteristics, and the stability of a globally significant carbon pool
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Organic matter (OM) in arctic permafrost ground contains about twice as much carbon (C) as is currently present in the atmosphere. Climate change is particularly strong in the Arctic, and could cause a considerable part of the OM in permafrost to thaw out, decompose, and be released as greenhouse gases; further enhancing global warming. The exact size of the northern circumpolar C pool remains unclear, and processes that control decomposition and mineralization rates are even more uncertain. Superimposed on the long-term release of C through microbial decomposition of OM in the gradually deepening active layer, is the rapid release of currently sequestered OM through geomorphological processes. This thesis considers the quantity, quality, and availability of permafrost C, and explores interactions and common controls.To better understand the potential effects of thawing permafrost, it is vital to: i) obtain more accurate size and distribution estimates of permafrost C stocks, and develop methods to accurately and efficiently implement these in models, ii) identify OM characteristics that control decomposition, specifically for permafrost material, and iii) determine and quantify key geomorphological processes that cause large amounts of OM to become available for rapid decomposition.Detailed C quantifications are valuable to increase our fundamental understanding of permafrost soil processes and C sequestration, but including high levels of heterogeneity in models is challenging. Simple upscaling tools based on e.g. elevation parameters (Paper I) can help to bridge the gap between detailed field studies and global C models.Permafrost OM quality is controlled by different factors than those commonly observed in temperate soils (without permafrost). We observed an unexpected (significant) correlation in upper permafrost samples, where material that is generally considered more recalcitrant showed the highest CO2 production rates per g C, indicating high lability (Paper II). In ancient Pleistocene permafrost, labile samples related significantly to OM that was enriched in decomposed microbial remains, whereas less-decomposed plant material was more stable (Paper III). Investigation of multiple incubation datasets revealed that the unusual relationship between %C and CO2 production occurred in contrasting field sites throughout the Arctic, indicating important permafrost-specific controls over OM quality (Paper IV). We discuss several possible explanations for the observed high lability of permafrost OM, such as a pool of labile dissolved organic C in the upper permafrost, or increased lability caused by past decomposition. In order to conclusively identify causal relationships, and to answer the question whether or not the same mechanisms control OM quality in different environments, further investigation of permafrost-specific OM quality is required.Geomorphology plays a key role in C reworking and OM decomposition. Vast amounts of OM can be released abruptly (e.g. in thaw slumps and thermokarst lakes, Paper II), resulting in C turnover that will likely outweigh decomposition through gradual active layer deepening. Climate change could enhance this rapid release of C, and changes in surface hydrology and increased fire activity are expected to become the largest contributors to C loss from permafrost regions. Together with C quantity and quality, availability through gradual and abrupt processes must be parameterized and included in models in order to accurately assess the potential permafrost C climate feedback.
|
|
